The present invention relates to electrographic copolymer compositions and electrographic imaging processes using such compositions.
Electrographic imaging and developing processes are well known in the art. These processes typically require forming a latent electrostatic charge image on an insulating electrographic element. The image is then made visible by treatment with an electrostatic toner composition or developer.
Developers generally comprise a polymeric toner material and carrier particles. The toner material may include a pigment such as carbon black or other colorant to darken or color images produced by the process. Carrier particles may be either a triboelectrically chargeable magnetic or non-magnetic material. Magnetic carrier particles include iron oxide, iron filings or powdered iron. Non-magnetic carrier particles may include glass beads or crystals of inorganic salts such as sodium or potassium chloride.
Typically, in an electrographic imaging process a developed image is formed on a photoconductive element and then transferred to a receiving sheet. The image is fixed by heating to fuse the transferred image. The toner material must therefore be capable of being fused under temperature conditions which will not result in charring, burning or other damage to the paper receiving sheet.
The transferred image is typically fixed by bringing the transfer sheet, bearing the transferred image, into contact with a heated fusing roller. During this process "toner offsetting" may result. Toner offsetting is the undesirable transfer of toner particles from the unfused toner image, carried on a receiving member (e.g., copy sheet), to the surface of a heated fusing member. The surface of the fusing member thus becomes contaminated with toner particles. The contaminating particles may be transferred to subsequent copy sheets upon further use of the fusing member. The typical contamination results in undesirable deposits of toner material on subsequent copy sheets in the form of a ghost image of previously fixed images, or in the image background of subsequent sheets. By extending a toner's softening range the range over which minimal toner offset occurs may be extended, eliminating or substantially decreasing the amount of overall toner offset.
The "softening range" as used herein is defined as the difference between (a) the temperature at which the plunger of a Schimadzu CFT 500 Flow Tester is displaced 4 millimeters, and (b) the temperature at the beginning of the rubbery plateau for a given sample. Softening ranges for typical non-crosslinked toner copolymers range from about 35 to about 45 Celsius temperature units. An extension of the softening range, by as little as two to three Celsius temperature units, produces significant differences in the copolymer's physical properties.
Many attempts have been made to extend the softening range or fusion range of toner copolymers. For example, U.S. Pat. No. RE. 31,072 proposes a solution to the problem of toner offsetting by providing a crosslinked toner with an extended fusion range. Although such a crosslinked toner may produce decreased toner offsetting, it would be advantageous to decrease toner offset with easily formulated, non-crosslinked toners having extended fusion, and hence, softening ranges.
Accordingly, it is an object of the invention to provide a toner for use with electrographic imaging processes which minimizes the potential occurrence of toner offsetting. Another object of the invention is to provide a method of reducing the offsetting problem in electrographic imaging processes. A further object is to provide a non-crosslinked polymeric toner material having an extended softening range for use with electrographic imaging processes. Other objects will be apparent to those of ordinary skill in the art upon reading the following disclosure.